Field emission electron gun utilizing means for protecting the field emission tip from high voltage discharges

ABSTRACT

A field emission electron gun comprises a field emission tip as its source of electrons. A first anode is spaced downstream from the tip and when a voltage is applied between the first anode and the tip, electrons from the tip are accelerated toward the first anode. An opening in the first anode limits the angular spread of the electron beam. A second anode is spaced downstream from the first anode and when a voltage is applied between the second anode and the tip, the energy level of the electrons at the image or specimen plane is controlled. The electrostatic field between the first and the second anode brings the electron beam into focus. For protecting the field emission tip against high voltage discharges, a third electrode in the form of a shield electrode surrounds the field emission tip and is maintained at or near the electrical potential of the tip. Within the shield is a fourth electrode or intermediate electrode which serves, when voltage is applied thereto as an electrode, to draw electrons from the tip and to restore or maintain normal operating conditions for the field emission electron gun. An ion-getter vacuum pump and a reactive sublimator vacuum pump are formed in the electron gun by evaporating a highly reactive element or getter material on the inner walls of the third electrode, which serves as a collector by inducing gas molecules which strike this surface to adhere thereto and to be imbedded therein. The inner walls of the third electrode react with reactive gases present in the region of the tip and the fourth electrode. The ion getter pump operates by ionizing residual gas molecules which are then impelled by electric fields and are imbedded under the coating of sublimed getter material. The primary electron beam from the tip strikes the surface of the fourth electrode, thereby causing reflected and secondary electrons to be emitted from the surface, which electrons form an electron cloud capable of ionizing molecules within the chamber. The electron cloud is formed and the ionized gas molecules are collected by applying the appropriate potentials to the electrodes in the gun assembly. The third electrode may be cooled by a liquid nitrogen cooling system, which functions as a cryogenic vacuum pump. This cooling system can also be used to cool the tip in order to reduce the tip flicker noise resulting in greater stability of electron emission.

waited States Patent Coates et al.

[451 July 18, 11972 [54] FIELD EMISSION ELECTRON GUN UTILIZING NEANS FORPROTECTDIG TEE FIELD ENHSSION TIP FROM EHGH VOLTAGE DISCHARGES [72]Inventors: Vincent J. Coates, Los Altos; Leonard M.

Welter, Saratoga, both of Calif.

[73] Assignee: American Optical Corporation,

Framingham, Mass. I

[22] Filed: June 15, 1970 [21] Appl. No.: 46,425

[52] U.S. Cl ..315/31, 250/495 A, 250/495 C, 250/495 GC, 250/495 R [51]Int. Cl ..HOlj 29/56 [58] field of Search ..250/495 A, 49.5 C, 49.5 GC,

[56] References Cited UNITED STATES PATENTS 2,363,359 11/1944 Ramo..250/49.5 3,394,874 7/1968 Marshall ..230/69 OTHER PUBLICATIONS Crewe,Electron Gun Using Field Emission Source, Review of Scientific Ins, Vol.39, No. 4

Primary Examiner-Benjamin R. Padgett Assistant Examiner-l M. PotenzaAttorney-William C. Nealon, Jeremiah J. Duggan, Robert J. Bird, BernardL. Sweeney and Joel Wall [5 7] ABSTRACT A field emission electron guncomprises a field emission tip as its source of electrons. A first anodeis spaced downstream from the tip and when a voltage is applied betweenthe first anode and the tip, electrons from the tip are acceleratedtoward the first anode. An opening in the first anode limits the angularspread of the electron beam. A second anode is spaced downstream fromthe first anode and when a voltage is applied between the second anodeand the tip, the energy level of the electrons at the image or specimenplane is controlled. The electrostatic field between the first and thesecond anode brings the electron beam into focus.

For protecting the field emission tip against high voltage discharges, athird electrode in the form of a shield electrode surrounds the fieldemission tip and is maintained at or near the electrical potential ofthe tip. Within the shield is a fourth electrode or intermediateelectrode which serves, when voltage is applied thereto as an electrode,to draw electrons from the tip and to restore or maintain normaloperating conditions for the field emission electron gun. An ion-gettervacuum pump and a reactive sublimator vacuum pump are formed in theelectron gun by evaporating a highly reactive element or getter materialon the inner walls of the third electrode, which serves as a collectorby inducing gas molecules which strike this surface to adhere theretoand to be imbedded therein. The inner walls of the third electrode reactwith reactive gases present in the region of the tip and the fourthelectrode. The ion getter pump operates by ionizing residual gasmolecules which are then impelled by electric fields and are imbeddedunder the coating of sublimed getter material. The primary electron beamfrom the tip strikes the surface of the fourth electrode, therebycausing reflected and secondary electrons to be emitted from thesurface, which electrons form an electron cloud capable of ionizingmolecules within the chamber. The electron cloud is formed and theionized gas molecules are collected by applying the appropriatepotentials to the electrodes in the gun assembly. The third electrodemay be cooled by a liquid nitrogen cooling system, which functions as aor ogenic vacuum pump. This cooling system can also be use to cool thetip in order to reduce the tip flicker noise resulting in greaterstability of electron emission.

37 Claims, 8 Drawing Figures United States Patem [151 3,678,333 Coateset a]. [451 July 18, 1972 PATENTED JUL 1 8 1912 SHEET 2 OF 5 V/ncenf J.Coafes Leonard M. Welter INVENTORS Attorney PATENTED SHEET u UF 53.678.833

Fig 5 Vincent J. Coo/es Leonard M. We/fer lNl/E/V 70/?5 AfforneyPATENTEDJuLmsn 3.678.333

sum 5 0F 5 V/ncent J. 60 ates F /'g 8 Leonard M. Welter INVENTORSAttorney FIELD EMISSION ELECTRON GUN UTILIZING MEANS FOR PROTECTING THEFIELD EMISSION TIP FROM HIGH VOLTAGE DISCHARGES BACKGROUND OF THEINVENTION The present invention relates in general to electron opticalsystems, and more particularly to a field emission electron gun.

Electron microscopes have heretofore comprised a field emission gunwhich included a field emission tip as its source of electrons. Spaceddownstream from the tip was a first anode. A voltage applied between thetip and the first anode drew electrons from the tip and accelerated theelectrons. Generally, the applied voltage was in the order of 5005,000volts. A second anode was spaced downstream from the first anode and avoltage applied between the tip and the second anode further acceleratedthe electrons. Generally, the applied voltage was in order of l,000l00,000 volts.

High voltage discharges taking place within the field emission gunsubjected the field emission tip to excessive potentials, which resultedin high tip current and subsequent melting of the field emission tip. Asa consequence thereof, the field emission tip failed prematurely andrequired frequent replacement.

Field emission tips have been disposed within chambers at ultra highvacuums in the order of Torr. or lower in order to operate withstability. Instability arises out of excessive gas molecules in thevicinity of the tip, which strike the tip or absorb to the tip surfaceto cause erratic electron emission. Vacuums in the order of 10: Torr. orlower have been produced by expensive, complex pumping systems.

Disclosures of field emission electron guns of the type above-describedare found in the article entitled Electron Gun Using A Field EmissionSource" by A. V. Crewe, D. N. Eggenburger, 1. Wall, and L. M. Welter,published in The Review of Scientific Instruments, Volume 39, Number 4,April 1968, Pages 576-583; U.S. Pat. to A. V. Crewe, No. 3,191,028,issued on June 22, I965, for Scanning Electron Microscope; an articleentitled A High-Resolution Scanning Electron Microscope by A. V. Crewe,.I. Wall and L. M.

Welter, published in the Journal of Applied Physics, Volume A fieldemission electron gun for an electron optical system having a fieldemission tip as a source of electrons. A shield electrode surroundingthe field emission tip is electrically connected to the field emissiontip for protecting the field emission tip against voltage discharges.

A further feature is that another electrode is disposed within theshield electrode to draw electrons from the tip and to maintain normaloperating conditions for the field emission gun.

From the foregoing arrangement, the delicate field emission tip is notsubjected to a high current flow when a voltage discharge occurs, and,therefore, the life expectancy of the field emission tip has beengreatly extended.

In order to help improve tip current stability by improving the localvacuum and by reducing ion damage to the tip, the shield electrode isheld at an electrical potential equal to or less positive than that ofthe field emission tip. Thus, there is a tendency for the shieldelectrode to attract positive ions away from the tip. The electronsproduced by the incident primary beam current from the tip at theelectrode within the shield electrode are in a negative electric field,and, hence, tend to form a constrained, high density electron cloudwithin the shield electrode. The electron cloud increases theprobability of ionizing gas molecules or other particles in the regioncontained by the shield electrode, and these ionized molecules are thenattracted toward the shield electrode.

In the exemplary embodiment of the present invention, a reactive elementor getter material is evaporated on the inner wall of the shieldelectrode to induce molecules impinging on the shield electrode toadhere thereto and to be imbedded in the inner wall of the shieldelectrode. In addition, when the beam of electrons is emitted from thetip, the inner wall of the shield electrode serves as an ion collectorto attract and capture in the reactive coating those gas molecules whichare ionized by the primary beam and by the electron cloud which the beamgenerates. Through this action, a getter-ion pump is present forproducing and maintaining ultra high vacuums by ionizing gas moleculeswhich are then impelled to the shield wall by electric fields and areburied there under the coating of sublimed getter material. A reactivesublimator pump is also present by inducing reactive gas molecules whichhappen to strike the shield electrode to stick there and to be buriedunder the getter material.

It has been found that the number of gas molecules liberated from theelectrode within the shield electrode increases as the current emittedfrom the field emission tip increases. The number of secondary andreflected electrons generated at the surface of the electrode within theshield electrode also increases as the current emitted from the fieldemission tip increases. Therefore, the pumping efficiency of the presentgetter-ion pump increases as the pumping requirements increase. Inaddition, the shield electrode may be cooled to improve the overallpumping efficiency of the vacuum system. This improvement occurs becauseof the increased pumping for the condensable gases and the moreefficient pumping of reactive gases. The same cooling system is thenused to cool the tip which reduces flicker noise and improves tipcurrent stability.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of anelectron optical system embodying the field emission gun of the presentinvention.

FIG. 2 is a front elevation view of the field emission gun incorporating therein the present invention.

. FIG. 3 is an enlarged top view of the field emission gun shown in FIG.2.

FIG. 4 is a vertical section view taken along line 44 of FIG. 3.

FIG. 5 is a partial vertical section view taken along line 55 of FIG. 3.

FIG. 6 is a horizontal section view taken along line 66 of FIG. 4.

FIG. 7 is a horizontal section view taken along line 7-7 of FIG. 4.

FIG. 8 is a horizontal section view taken along line 8-8 of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Illustrated in FIG. 1 is anelectron Optical system in the form of a scanning electron microscope 10which embodies therein a field emission gun 15 of the present invention.The field emission gun 15 produces from its electron source a bright,focused spot of electrons at its image plane to illuminate a specimen20. The focused spot is scanned by means of a deflection system andstigmator 25. Information about the specimen 20 is obtained by detectingtransmitted electrons, secondary electrons, reflected electrons,absorbed electrons, photons, or X-rays, any or all of which aregenerated by the incident electron beam. Detectors 30 and 30a are oftenused to detect one of these signals which is then used to modulate theintensity of a synchronously scanned display tube 35 with a sweepgenerator 36 to form an image of the specimen 20.

In FIGS. 2-8 is illustrated the field emission gun 15 of the presentinvention, which comprises a substantially cylindrical housing 40defining a vacuum chamber 41 (FIGS. 4 and 5). Disposed within the vacuumchamber 41 and the high vacuum area 41a thereof is a source ofelectrons, such as a field emission tip 45. The field emission tip 45 isan etched tip which does not employ any filament voltage or power toemit electrons. The cold field emission tip 45 is very small indiameter, such as 1,000A.

For supporting the field emission tip 45 for movement along the x, y and2, planes and for the removal of the field emission tip 45 forreplacement, a tip support assembly 50 comprises a V-shaped mount 51,which has the tip 45 attached thereto. The V-shaped mount 51 is crimpedbetween electrodes 52 and 53 in fixed relation. An insulator disc 54 forthe tip 45 receives the electrodes 52 and 53 and the electrodes 52 and53 are secured to the insulator disc 54 by nuts 56 and 57. Theelectrodes 52 and 53 are received as prongs by the lower end of a highvoltage insulator post 55. Nuts 56 and 57 on the electrodes 52 and 53limit the extent of the entry of the electrodes 52 and 53 into thedistal end of the post 55. From this arrangement, the tip 45, the disc54 and the electrodes 52 and 53 are in the form of a plug which isdetachably secured to the distal end of the post 55. For replacing thetip 45, the plug is detached as a unit and replaced as a unit.

The upper end of the insulator post 55 is fixedly secured to a metalshaft 60 having an end cap 60a with a depending flange fixed within atapped hole of the upper end of the post 55. Mounted on the cap 60a isfixed relation is a bellows 61. The bellows 61 are welded to the cap 60ato form a vacuum wall or seal with the high vacuum chamber whilepermitting axial and transverse movement of the tip support assembly 50.The bellows 61 are welded directly to an end cap 62 to complete thevacuum seal. Surrounding the shaft 60 is a sleeve 63 (FIG. 4) that isreceived with the shaft 60 by an opening 64 in the cap 62. Spaced fromthe cap 62 is a support plate 65 that has a suitable opening therein forreceiving the shaft 60 and the sleeve 63.

To tilt the plate 65 relative to the cap 62 in two different planes,screws 66 engage the cap 62 at their free ends and are in threadedengagement with the plate 65. This produces x and y motion of the tip bypivoting sleeve 63 on pivot bearing 67 fixed to the cap 62 by a pivotbearing mount. Thus, as sleeve 63 pivots, so does shaft 60 and tipsupport assembly 50 resulting in x and y motion of the tip. Springs, notshown, cooperate with the screws 66 and 67 to maintain the plate 65 inits adjusted position relative to the cap 62. A knob 68 is in threadedengagement with the upper end of the shaft 60 and is rotatable at afixed height. Thrust bearings 69 permit the rotation of the knob 68relative to the plate 65. Thus, rotation of the knob 68 imparts axial orvertical movement to the shaft 60 without rotating the shaft 60.Vertical movement of the shaft 60 imparts a vertical movement to the tip45 through the post 55 and the electrodes 52 and 53. Movement of the tip45 in the vertical direction serves as a means for focusing the electronbeam emitted from the tip 45.

Spaced downstream from the tip 45 in the low vacuum area 41b of thevacuum chamber 41 is a first anode 75 of a first focusing electrodeformed with an outer cylindrical wall and a central opening 84. Thefirst anode 75 is well-known and is described in the aforementionedarticle by A. V. Crewe et al. entitled Electron Gun Using A FieldEmission Source.

Spaced downstream from the first electrode 75 is a second anode orsecond focusing electrode 85, which has an annular cylindrical wall andan axial cylindrical wall. An annular insulating support 86 with astepped inwardly facing configuration supports the first anode 75 on theshoulder thereof and is fixed to the second anode 85 to be supportedthereby.

When a positive voltage V, is applied between the tip 45 and the secondanode 85, with the positive voltage brought to second anode 85, theelectrons emitted from the tip 45 are further accelerated. 1n thetypical embodiment, the voltage V,, is equal to 20,000 volts. The secondanode 85 is attached to the chamber of the gun and in turn to the morepositive terminal of the V,, supply. The second anode 85 is well-knownand is described in the aforementioned article by A. V. Crewe et al.entitled Electron Gun Using A Field Emission Source. It is the secondanode 85 that controls the energy level of the electrons impinging onthe specimen 20.

It has been found that the tip 45 fails, when subjected to excessivedischarge voltages, because of the high tip current which then flows andmelts the end of the tip. According to the present invention, a thirdelectrode 90 or shield electrode is disposed within the high vacuum area410 of the vacuum chamber 41 to reduce premature failure of the tip 45and to extend the life thereof.

The shield electrode has an outer cylindrical wall with an upper endwall of a reduced diameter opening and with an inwardly turned, arcuatecentral wall. As shown in the drawings, the shield electrode 90surrounds the tip 45 and is connected to an output terminal 77 of theinsulator 78 (FIG. 4). Thus, the third electrode 90 is substantially atthe same electrical potential as is the tip 45 and is electricallyconnected thereto through a low impedance. It is the shield electrode 90that protects the tip 45 from excessive voltage discharges from thefirst and second anodes, and, hence, the tip 45 is not exposed to highvoltage transients. As a consequence thereof, the life of the tip 45 isextended. In the exemplary embodiment, the shield electrode 90 is madeof mu metal, which provides a shield against stray outside magneticfields that tend to deflect the electron beam emitted by the tip 45.

A fourth electrode is disposed within the shield electrode 90. When avoltage is applied to the fourth electrode 95, electrons are drawn fromthe tip 45. It is the fourth electrode 95 that restores the electronemission gun 15 to normal operation or maintains the operation of theelectron gun normal, when a shield electrode is employed to protect thetip 45 from excessive voltage transients. The source of high voltage,herein referred to as V,, is in the typical embodiment in the order ofapositive SOD-3,000 volts. When the voltage V, is applied between the tip45 and the fourth electrode 95, electrons are drawn from the tip 45 andare accelerated toward the first anode 75. The fourth electrode 95 isconnected to the first anode through a current limiting resistor.

For this purpose, a suitable high voltage supply 76 has its low voltageoutput connected to a terminal 77 of the high voltage insulator 78. Theterminal 77 is also connected to the electrode 52, of the tip mountingassembly 50 over conductor 81 (FIG. 7) as well as to the shieldelectrode 90. A terminal 77' is connected to the electrode 53 over theconductor 80. The terminal 79, which is the high voltage V, output, isconnected over a conductor 82 to the fourth electrode 95 and, in turn,is connected to the first anode 75 through a voltage dropping resistor83 (FIG. 4). This voltage dropping resistor 83 serves to maintain thefourth electrode 95 at normal V, voltage when a high voltage dischargecauses the first anode 75 to are to the second anode 85. The beam ofelectrons eminating from the tip 45 passes through a small opening inthe fourth electrode 95, which is narrower than the beam of electronsand thereby controls the angular spread of the beam of electrons passingon to the first anode 75 and through its opening 84. Note that theelectrons that are incident on the fourth electrode 95 and generate theelectron cloud, initiate the pumping action of the getter-ion of theshield electrode 90.

It is to be observed that the assembly for adjusting the location of thetip 45, which has movement in the x, y and 2 planes, is separate andapart from the high voltage insulator 78. When the high voltageinsulator established its connections through the tip support assembly50, the tip 45 was subjected to vibrations. Vibrations on the tip 45tend to cause degradation in the resolution of the scanning electronmicroscope 10. Thus, permanent connections are established with theelectrodes 52 and 53 through the flexible conductors 80 and 81, yetenabling the tip 45 to be moved within the vacuum chamber 41 in the x, yand 2 planes.

it is known that excessive gas molecules in the vicinity of the tip 45causes erratic or noisy electron emission from the tip 45. This resultsin poor resolution for a scanning electron microscope. Thus, it wascustomary to employ powerful and expensive pumping systems to obtain ahigh vacuum area for the field emission tip.

According to the present invention, a very reactive element, or gettermaterial, such as titanium, is evaporated from an annular filament 91onto the inner wall of the shield electrode 90. One-end of the filament91 is connected to the terminal 77 and the other end of the filament 91is connected to the voltage V The evaporation of the titanium on theinner wall of the shield electrode 90 may be either a continuous processor a periodic process. The titanium on the wall of the shield electrode90 will react with and imbed therein reactive gasses that arrive at itssurface. It therefore helps produce ultra high vacuums by collecting andburying gas molecules under its coating of sublimed getter material.

However, electrons emitted from the tip 45 are accelerated toward andstrike the fourth electrode 95 causing large local pressure increases byliberation of positive ions, negative ions and neutral molecules fromthe surface of the fourth electrode 95. In addition, secondaryelectrons, reflected electrons and energetic X-rays are emitted. Theliberation of the molecules which enter from other places would causethe tip to contaminate and the electron emission from the tip to becomeunstable. Such a condition would impair the operation of the electronemission gun by making it noisy, erratic and subject to prematurefailure. Further, instability may be caused by some positive ions whichare accelerated back to the field emission tip and damage it.

The shield electrode 90 of the present invention is held at anelectrostatic potential which attracts these positive ions away from thetip 45 as an ion collector. The secondary and reflected electronsproduced at the surface of the fourth electrode 95 are within a negativeelectrostatic field and tend to form a high density electron cloudconfined within the shield of the third electrode 90. The electron cloudincreases the probability of ionizing any gas molecules in the highvacuum area which are then attracted toward the shield electrode 90.Thus, a local getter-ion pump operation is established in conjunctionwith the sublimation pumping.

Although the above-described operation can be performed with the shieldelectrode 90 at ambient temperatures, the pumping efficiency of themolecules in the titanium covered shield electrode 90 can be improved bycooling the shield electrode 90. Toward this end, the shield electrode90 is cooled by a liquid nitrogen cooling system 100. The cooling system100 comprises a container 101 of toroidal configuration for storingliquid nitrogen, which container 101 is connected to the shieldelectrode 90 through a beryllium oxide insulator 102 (FIG. 4) also of anannular configuration. The beryllium oxide insulator 102 is not only agood high voltage insulator but also has a high thermal conductance.Therefore the shield electrode 90 because of its strong negativeelectrostatic field not only produces the electron cloud of secondaryelectrons and holds back the secondary electrons from being acceleratedonto the inner wall of the housing 40, which defines the vacuum chamber41, thus obviating further outgassing and pressure increases. but alsois part of a cryogenic pump operation performed in the high vacuum area41 a of the vacuum chamber 41.

Through the getter-ion pump operation of the shield electrode 90 and thecryogenic pump operation of the nitrogen cooling system 100, a highvacuum in the order of Torr. is created in the vacuum chamber 41 withoutadditional expensive, independent vacuum pumps.

Electrons emitted from the tip 45 travel through the opening in thefourth electrode 95, through the opening 84 of the first electrode 75,through a similar opening 106 in the second electrode 85, through a beamguide tube 107 disposed axial of the deflection system and stigmator 25to impinge on the specimen 20. For controlling the cross-sectional areaof the electron beam, an aperture selector plate 110 is disposed betweenthe second anode 95 and the deflecting system and stigmator 25. Theaperture selector plate 110 includes a series of openings varying indiameter. In the exemplary embodiment, there are four such holes rangingin diameter from 25 to 250 microns.

By sliding the plate 110 horizontally, a selected aperture is alignedwith the tube 107 and the opening 106 of the second anode 85 to controlthe size of the electron beam advancing therethrough. For this purpose,a shaft 111 is secured to the plate 110 for imparting a horizontalmovement thereto. A

housing 112 is fixed in sealing engagement with the housing 40 throughO-rings 113. O-rings 114 provide a seal between the shaft 111 and thehousing 112. Seated in the housing 114 and surrounding the shaft 111 arebellows 115, which are welded thereto to permit the shaft 111 to haveaxial movement while maintaining a fluid seal. Outside the housing 40,the shaft 111 is threaded to knobs 116 and 117. Suitable bearings 118enable the knobs 116 and 117 to be rotated relative to a cap 120 for thehousing 112. Thus, rotation of the knob 117 imparts reciprocal movementto the shaft 111 without rotating the shaft 111 to align a selectiveopening of the plate 111 with the path of travel of the electron beam tocontrol the diameter thereof. Knob 116 is an eccentric that impartsangular transverse motion to the plate 110 about the O-ring 114 whichserves as a pivot point for the transverse motion.

When the tip 45 is changed, the chamber 41 is at atmospheric pressure.To facilitate the reduction of pressure in the chamber 41 fromatmospheric to a high vacuum, the chamber 41 is exposed to theatmosphere by fully retracting the shaft 111 through the knob 117. Whenthis is done, a vent in the housing 112 communicates through an opening126 with the chamber 41 by the O-rings 114 advancing into the increaseddiameter portion of the housing 112. By returning the shaft 111 to itsinitial position, the valve is closed and the chamber 41 is not ventedthrough the opening 125, since the O-rings 114 occupy the position shownin FIG. 4. Differential pumping occurs between the high vacuum in thechamber 41a and the chamber 41b through the opening 84 on the firstanode 75.

For isolating the chamber 41 from atmospheric pressure while thespecimen 20 is being changed, a valve 130 (FIG. 4) with an opening 131formed in a valve shaft 143 is slidable in a horizontal direction so asto remove the opening 131 from alignment with the lower end of the tube107 to block the same. O-ring 143 seals the end of the opening 107 whenthe valve 130 is closed. After the specimen is changed, the opening 131is realigned with the bottom of the tube 107. A suitable vacuum pump 135is attached to the lower wall of the emission gun 15 and draws out theair under atmospheric pressure in the chamber 41 until the chamber isunder a vacuum in the order of 10" or 10 Torr.

For sliding the valve 130 into and out of venting position, a valvehousing 136 is fixed to the housing 40. The housing 136 is sealed by acap 137 and O-rings 138. O-rings 139 provide a seal between the housing136 and the housing 40. Bellows 141 welded to the cap 137 and a sleeve142 surrounding the valve shaft 143 permit rectilinear motion of thevalve shaft 143 without breaking the seal. A knob 145 in threadedengagement with the valve shaft 143 is rotated to impart a rectilinearmovement to the shaft 143 without rotating the same.

After the chamber 41 has been evacuated to about 10 or 10" Torr., theshaft 111 is moved to close off the vent 125, thereby closing thedifferential valve and the opening 131 is aligned with the tube 107through the movement of the valve shaft 143. When the opening 131 isaligned with the tube 107, electrons are permitted to impinge on thespecimen 20. At this time, the vacuum in the chamber 41 is maintained bythe cryogenic pump 100, the titanium sublimator and ion getter pump 91,and the vacuum in chamber 41b is maintained by the pump 135 at about 10Torr. These pumps are adequate to evacuate the high vacuum chamber to l0Torr. This vacuum provides stable operating conditions for the tip 45.The differential pumping now takes place through the opening 84 of thefirst anode 75, since this is the only path for establishingcommunications between the high vacuum chamber 41a and a low vacuumchamber 41b below the opening 84.

In changing a specimen, the valve shaft 143 is employed to isolate thehigh vacuum chamber 41a of the field emission gun 15 keeping it at ahigh vacuum, while the low vacuum chamber 41b is pressurized toatmospheric pressure.

The shield electrode 90 has its cylindrical wall connected to its baseby suitable means such as a hinge contact 151. In addition thereto, thehousing 40 has separable sections in its cylindrical wall at the O-ring113, which are suitably connected through a suitable hinge 152. When itis desired to replace the tip 45, the gun 15 is pivoted at the hinges151 and 152 to provide access for the replacement of the plug mountingfor the emission tip 45.

We claim:

1. A field emission electron gun comprising:

a. a housing defining a vacuum chamber;

b. a field emission tip disposed in said vacuum chamber for providing asource of electrons;

c. a shield electrode disposed in said vacuum chamber and defining anarea in which said field emission tip is disposed for preventingexcessive voltage discharges to said field emission tip;

(1. anode means operatively associated with said field emission tip andsaid shield electrode for establishing an electric field operativelyeffective within said area for controlling said source of electrons whendisposed within said shield electrode area; and

e. voltage means interconnecting said field emission tip and said shieldelectrode to said anode means for applying electrical potential theretoto protect said field emission tip from excessive discharge voltages andto establish said electric control field.

2. A field emission electron gun as claimed in claim 1 wherein saidanode means includes an intermediate electrode disposed in said areadefined by said shield electrode, said voltage means being connected tosaid intermediate electrode disposed in said area defined by said shieldelectrode to apply a potential thereto for drawing electrons from saidfield emission tip.

3. A field emission gun as claimed in claim 2 in which said intermediateelectrode disposed in the area defined by said shield electrodemaintains normal operating conditions for said field emission gun.

4. A field emission gun as claimed in claim 2 including means locatedwithin said area for maintaining a low pressure.

5. A field emission gun as claimed in claim 4 wherein a getter materiallocated within said area captures particles thereby maintaining said lowpressure.

6. A field emission gun as claimed in claim 5 wherein said gettermaterial is disposed on said shield electrode and said particlesimpinging on the inner surface thereof adhere thereto and are imbeddedtherein for producing said low pressure.

7. A field emission electron gun as claimed in claim 4 and comprising acontainer of coolant disposed in said vacuum chamber and communicatingwith said shield electrode for cooling said shield electrode forproducing said low pressure and for reducing tip flicker noise.

8. A field emission gun as claimed in claim 4 wherein said means formaintaining a low pressure includes electrons from said field emissiontip and secondary and reflected electrons from said intermediateelectrode which ionize particles, and means for capturing said ionizedparticles.

9. A field emission gun as claimed in claim 8 and comprising a containerof coolant disposed in said vacuum chamber and communicating with saidshield electrode for cooling said shield electrode and for producingsaid low pressure.

10. A field emission gun as claimed in claim 4 wherein said means formaintaining a low pressure includes a filament for heating said gun tominimize outgassing of said vacuum chamber.

11. A field emission gun as claimed in claim 6 wherein a sublimationfilament associated with said shield electrode produces a sublimedmaterial on said inner surface thereby forming said getter material.

12. A field emission gun as claimed in claim 6 wherein electrons fromsaid field emission tip and secondary and reflected electrons from saidintermediate electrode ionize said particles, said ions striking saidinner surface and adhereing thereto and imbedded therein for producingsaid low pressure, said electrical potential of said shield electrodebeing selected so as to attract said ions.

13. A field emission electron gun as claimed in claim 7 wherein athermal conductive electrical insulator is disposed in said vacuumchamber between said shield electrode and said container of liquidcoolant.

14. A field emission electron gun as claimed in claim 4 and comprisingan adjustable support on said housing projecting into said vacuumchamber with said field emission tip attached thereto for adjustablypositioning said field emission tip within said vacuum chamber.

15. A field emission electron gun as claimed in claim 14 wherein saidfield emission tip is removably attached to said adjustable support.

16. A field emission electron gun as claimed in claim 15 wherein saidfield emission tip includes electrodes that are plugable elements forremovably attaching said field emission tip to said adjustable support.

17. A field emission electron gun as claimed in claim 14 wherein saidvoltage means includes a high voltage terminal which is supported bysaid housing at a location removed from said adjustable support.

18. A field emission electron gun as claimed in claim 4 wherein saidhousing includes separable sections pivotally attached for gainingaccess to said field emission tip, said shield electrode being formedwith separable sections pivotally at.- tached for separation in responseto said housing sections being separated for providing access to saidfield emission tip.

19. A scanning electron microscope field emission electron guncomprising:

a. a housing defining a vacuum chamber;

b. a field emission tip disposed in said vacuum chamber for providing asource of electrons;

c. a first anode disposed in said chamber and spaced downstream fromsaid field emission tip;

d. a second anode disposed in said chamber and spaced downstream fromsaid first anode;

e. a shield electrode disposed in said chamber and defining an area inwhich said field emission tip is disposed for preventing excessivevoltage discharges to said field emission tip;

f. an intermediate electrode located within said area; and

g. voltage means connected to said field emission tip, said first anode,said second anode, said shield electrode and said intermediate electrodefor applying a voltage between said field emission tip and saidintermediate electrode to accelerate electrons toward said first anodeand for applying a voltage between said intermediate electrode, saidfirst anode and said second anode to accelerate electrons toward saidsecond anode and for applying potentials to said shield electrode andsaid intermediate electrode for protecting said field emission tipagainst excessive voltage discharges, said shield and said intermediateelectrodes and said first and second anodes having apertures alignedwith said field emission tip to permit passage of electrons forming abeam.

20. A field emission electron gun as claimed in claim 19 wherein saidshield electrode is at the same electrical potential as said fieldemission tip.

21. A field emission electron gun as claimed in claim 19 wherein saidintermediate electrode is connected to said first anode through aresistor to prevent discharge from said intermediate electrode isconnected to said first anode through a resistor to prevent dischargefrom said intermediate electrode to said field emission tip.

22. A field emission electron gun as claimed in claim 21 wherein saidshield electrode is connected to said tip through a low impedance.

23. A field emission electron gun as claimed in claim 19 wherein saidvacuum chamber has a higher vacuum section and a lower vacuum section,said field emission tip, said shield electrode and said intermediateelectrode being disposed in said higher vacuum section, said first anodeand said second anode being disposed in said lower vacuum section.

24. A field emission electron gun as claimed in claim 23 and comprisinga valve mounted on said housing between said high and low vacuumsections for venting fluid under atmospheric pressure in said vacuumchamber after said field emission tip has been replaced.

25. A field emission electron gun as claimed in claim 23 and comprisinga valve mounted on said housing for isolating said lower vacuum chamberfrom fluid under atmospheric pressure when a portion of said housingadjacent said lower vacuum chamber is vented to the atmosphere.

26. A field emission electron gun as claimed in claim 1 and comprising acontainer of liquid coolant disposed in said vacuum chamber andcommunicating with said field emission tip for reducing flicker noise.

27. A field emission electron gun as claimed in claim 3 wherein saidshield electrode is at substantially the same potential as said fieldemission tip.

28. A field emission electron gun as claimed in claim 27 wherein saidshield electrode is electrically connected to said tip through a lowimpedance.

29. The field emission electron gun of claim 2 wherein said anode meansincludes first and second anodes placed downstream of said intermediateelectrode for accelerating and focusing emitted electrons and saidvoltage means includes impedance means interconnecting said intermediateelectrode and said first anode for limiting said intermediate anodevoltage with respect to said field emission tip during dischargeconditions.

30. The field emission electron gun of claim 29 wherein said impedancemeans comprises a substantially pure resistive element. 7

31. A field emission gun as claimed in claim 19 including means locatedwithin said area for maintaining a low pressure.

32. The field emission electron gun of claim 31 including a sublimationfilament associated with said shield electrode producing sublimedmaterial which combines with reactive particles on an inner surface ofsaid shield electrode thereby forming a getter material on said innersurface and wherein electrons from said field emission tip and secondaryand reflected electrons from said intermediate electrode ionizeparticles, said ions striking said inner surface and adhering theretoand imbedded therein for producing said low pressure.

33. The field emission gun of claim 12 wherein said ionized particles ofreactive contaminant constituents reactively combine with said gettermaterial and said ionized particles of nonreactive contaminantconstituents are buried under subsequently sublimed getter material.

34. The field emission gun of claim 33 wherein said filament comprises atitanium-coated filament mounted on the periphery of an inwardly turnedarcuate wall of said shield electrode surrounding said field emissiontip, said filament subliming titanium when energized by a predeterminedlevel of power.

35. The field emission gun of claim 33 wherein particles sputtered fromsaid tip and said electrodes are imbedded in said getter material.

36. A field emission electron gun comprising:

a housing defining a vacuum chamber;

a field emission tip disposed in said vacuum chamber for providing asource of electrons;

electrode means disposed in said vacuum chamber and defining an area inwhich said field emission tip is disposed; and

means operatively associated with said electrode means and said fieldemission tip for dynamically vacuum pumping said area contiguous withsaid field emission tip.

37. A field emission gun as claimed in claim 1 including means fordynamically vacuum pumping said area contiguous with said field emissiontip.

Patent No. 6780333 Dat d July 18 1972 lnventofls) Vincent Jo Coates andLeonard M. Welter It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

In the specification:

Column 3,, line 20, after "60a" and before "fixed" change "is" to in---In the claims:

Claim 21, lines 4 and 5, after "electrode' (line 4) delete "is connectedto said first anode through a resistor to prevent discharge from saidintermediate electrode",

Signed and sealed this 19th day of February 1974.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. C, MARSHALL DANN Attestlng Officer Commissioner ofPatents FORM Po-1o50 (10-69) USCOMM-DC 60376-P69 U.S. GOVERNMENTPRINTING OFFICE: 1968 0-366-334.

Patent No. 7 33 Dated July 18, 1972 Inventm-(s) Vincent J. Coates andLeonard M. Welter It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

In the specification:

Column 3 line 20, after "60a" and beiore "fixed" change "is" to -in-.

In the claims:

Claim 21, lines 4 and 5. after "electrode" (line 4) delete "is connectedto said first anode through a resistor to prevent discharge from saidintermediate electrode".

Signed and sealed this 19th day of February-1974.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. I C, MARSHALL DANN Attestlng Officer Commissionerof Patents FORM PC4050 (10-69) USCOMM DC 603764369 w u.s. GOVERNMENTPRINTING OFFICE 1959 0-366-334,

1. A field emission electron gun comprising: a. a housing defining avacuum chamber; b. a field emission tip disposed in said vacuum chamberfor providing a source of electrons; c. a shield electrode disposed insaid vacuum chamber and defining an area in which said field emissiontip is disposed for preventing excessive voltage discharges to saidfield emission tip; d. anode means operatively associated with saidfield emission tip and said shield electrode for establishing anelectric field operatively effective within said area for controllingsaid source of electrons when disposed within said shield electrodearea; and e. voltage means interconnecting said field emission tip andsaid shield electrode to said anode means for applying electricalpotential thereto to protect said field emission tip from excessivedischarge voltages and to establish said electric control field.
 2. Afield emission electron gun as claimed in claim 1 wherein said anodemeans includes an intermediate electrode disposed in said area definedby said shield electrode, said voltage means being connected to saidintermediate electrode disposed in said area defined by said shieldelectrode to apply a potential thereto for drawing electrons from saidfield emission tip.
 3. A field emission gun as claimed in claim 2 inwhich said intermediate electrode disposed in the area defined by saidshield electrode maintains normal operating conditions for said fieldemission gun.
 4. A field emission gun as claimed in claim 2 includingmeans located within said area for maintaining a low pressure.
 5. Afield emission gun as claimed in claim 4 wherein a getter materiallocated within said area captures particles thereby maintaining said lowpressure.
 6. A field emission gun as claimed in claim 5 wherein saidgetter material is disposed on said shield electrode and said particlesimpinging on the inner surface thereof adhere thereto and are imbeddedtherein for producing said low pressure.
 7. A field emission electrongun as claimed in claim 4 and comprising a container of coolant disposedin said vacuum chamber and communicating with said shield electrode forcooling said shield electrode for producing said low pressure and forreducing tip flicker noise.
 8. A field emission gun as claimed in claim4 wherein said means for maintaining a low pressure includes electronsfrom said field emission tip and secondary and reflected electrons fromsaid intermediate electrode which ionize particles, and means forcapturing said ionized particles.
 9. A field emission gun as claimed inclaim 8 and comprising a container of coolant disposed in said vacuumchamber and communicating with said shield electrode for cooling saidshield electrode and for producing said low pressure.
 10. A fieldemission gun as claimed in claim 4 wherein saiD means for maintaining alow pressure includes a filament for heating said gun to minimizeoutgassing of said vacuum chamber.
 11. A field emission gun as claimedin claim 6 wherein a sublimation filament associated with said shieldelectrode produces a sublimed material on said inner surface therebyforming said getter material.
 12. A field emission gun as claimed inclaim 6 wherein electrons from said field emission tip and secondary andreflected electrons from said intermediate electrode ionize saidparticles, said ions striking said inner surface and adhereing theretoand imbedded therein for producing said low pressure, said electricalpotential of said shield electrode being selected so as to attract saidions.
 13. A field emission electron gun as claimed in claim 7 wherein athermal conductive electrical insulator is disposed in said vacuumchamber between said shield electrode and said container of liquidcoolant.
 14. A field emission electron gun as claimed in claim 4 andcomprising an adjustable support on said housing projecting into saidvacuum chamber with said field emission tip attached thereto foradjustably positioning said field emission tip within said vacuumchamber.
 15. A field emission electron gun as claimed in claim 14wherein said field emission tip is removably attached to said adjustablesupport.
 16. A field emission electron gun as claimed in claim 15wherein said field emission tip includes electrodes that are plugableelements for removably attaching said field emission tip to saidadjustable support.
 17. A field emission electron gun as claimed inclaim 14 wherein said voltage means includes a high voltage terminalwhich is supported by said housing at a location removed from saidadjustable support.
 18. A field emission electron gun as claimed inclaim 4 wherein said housing includes separable sections pivotallyattached for gaining access to said field emission tip, said shieldelectrode being formed with separable sections pivotally attached forseparation in response to said housing sections being separated forproviding access to said field emission tip.
 19. A scanning electronmicroscope field emission electron gun comprising: a. a housing defininga vacuum chamber; b. a field emission tip disposed in said vacuumchamber for providing a source of electrons; c. a first anode disposedin said chamber and spaced downstream from said field emission tip; d. asecond anode disposed in said chamber and spaced downstream from saidfirst anode; e. a shield electrode disposed in said chamber and definingan area in which said field emission tip is disposed for preventingexcessive voltage discharges to said field emission tip; f. anintermediate electrode located within said area; and g. voltage meansconnected to said field emission tip, said first anode, said secondanode, said shield electrode and said intermediate electrode forapplying a voltage between said field emission tip and said intermediateelectrode to accelerate electrons toward said first anode and forapplying a voltage between said intermediate electrode, said first anodeand said second anode to accelerate electrons toward said second anodeand for applying potentials to said shield electrode and saidintermediate electrode for protecting said field emission tip againstexcessive voltage discharges, said shield and said intermediateelectrodes and said first and second anodes having apertures alignedwith said field emission tip to permit passage of electrons forming abeam.
 20. A field emission electron gun as claimed in claim 19 whereinsaid shield electrode is at the same electrical potential as said fieldemission tip.
 21. A field emission electron gun as claimed in claim 19wherein said intermediate electrode is connected to said first anodethrough a resistor to prevent discharge from said intermediate electrodeis connected to said first anode through a resistor to prevent dischargefrom said intermediate electrode to said field emission tip.
 22. A fieldemission electron gun as claimed in claim 21 wherein said shieldelectrode is connected to said tip through a low impedance.
 23. A fieldemission electron gun as claimed in claim 19 wherein said vacuum chamberhas a higher vacuum section and a lower vacuum section, said fieldemission tip, said shield electrode and said intermediate electrodebeing disposed in said higher vacuum section, said first anode and saidsecond anode being disposed in said lower vacuum section.
 24. A fieldemission electron gun as claimed in claim 23 and comprising a valvemounted on said housing between said high and low vacuum sections forventing fluid under atmospheric pressure in said vacuum chamber aftersaid field emission tip has been replaced.
 25. A field emission electrongun as claimed in claim 23 and comprising a valve mounted on saidhousing for isolating said lower vacuum chamber from fluid underatmospheric pressure when a portion of said housing adjacent said lowervacuum chamber is vented to the atmosphere.
 26. A field emissionelectron gun as claimed in claim 1 and comprising a container of liquidcoolant disposed in said vacuum chamber and communicating with saidfield emission tip for reducing flicker noise.
 27. A field emissionelectron gun as claimed in claim 3 wherein said shield electrode is atsubstantially the same potential as said field emission tip.
 28. A fieldemission electron gun as claimed in claim 27 wherein said shieldelectrode is electrically connected to said tip through a low impedance.29. The field emission electron gun of claim 2 wherein said anode meansincludes first and second anodes placed downstream of said intermediateelectrode for accelerating and focusing emitted electrons and saidvoltage means includes impedance means interconnecting said intermediateelectrode and said first anode for limiting said intermediate anodevoltage with respect to said field emission tip during dischargeconditions.
 30. The field emission electron gun of claim 29 wherein saidimpedance means comprises a substantially pure resistive element.
 31. Afield emission gun as claimed in claim 19 including means located withinsaid area for maintaining a low pressure.
 32. The field emissionelectron gun of claim 31 including a sublimation filament associatedwith said shield electrode producing sublimed material which combineswith reactive particles on an inner surface of said shield electrodethereby forming a getter material on said inner surface and whereinelectrons from said field emission tip and secondary and reflectedelectrons from said intermediate electrode ionize particles, said ionsstriking said inner surface and adhering thereto and imbedded thereinfor producing said low pressure.
 33. The field emission gun of claim 12wherein said ionized particles of reactive contaminant constituentsreactively combine with said getter material and said ionized particlesof non-reactive contaminant constituents are buried under subsequentlysublimed getter material.
 34. The field emission gun of claim 33 whereinsaid filament comprises a titanium-coated filament mounted on theperiphery of an inwardly turned arcuate wall of said shield electrodesurrounding said field emission tip, said filament subliming titaniumwhen energized by a predetermined level of power.
 35. The field emissiongun of claim 33 wherein particles sputtered from said tip and saidelectrodes are imbedded in said getter material.
 36. A field emissionelectron gun comprising: a housing defining a vacuum chamber; a fieldemission tip disposed in said vacuum chamber for providing a source ofelectrons; electrode means disposed in said vacuum chamber and definingan area in which said field emission tip is disposed; and meansoperatively associated with said electrode means and said field emissiontip for dynamically vacuum pumping said area contiguous with said fieldemission tip.
 37. A field emission gun aS claimed in claim 1 includingmeans for dynamically vacuum pumping said area contiguous with saidfield emission tip.